A pathologist reviewing histological specimens from two patients with inflammatory bowel disease notes the following findings. Which statement about the pathological features of Crohn disease and ulcerative colitis is NOT correct?

Explanation

## Pathological Features: Crohn Disease vs Ulcerative Colitis **Key Point:** The incorrect statement is option 2, which reverses the transmural involvement pattern. Crohn disease (not ulcerative colitis) is characterized by transmural inflammation. ### Depth of Inflammation: The Critical Distinction **High-Yield:** Transmural involvement is the **defining pathological feature of Crohn disease**. Ulcerative colitis is limited to the mucosa and submucosa by definition. ```mermaid flowchart TD A[Inflammatory Bowel Disease]:::outcome --> B{Depth of inflammation?}:::decision B -->|Mucosa + Submucosa only| C[Ulcerative Colitis]:::outcome B -->|Transmural all layers| D[Crohn Disease]:::outcome C --> E[Crypt abscess<br/>Surface ulceration<br/>Continuous from rectum]:::action D --> F[Fissuring ulcers<br/>Fistulas & strictures<br/>Skip lesions]:::action ``` ### Comprehensive Comparison Table | Pathological Feature | Crohn Disease | Ulcerative Colitis | |---------------------|---------------|-------------------| | **Transmural inflammation** | ✓ Yes (defining) | ✗ No (mucosa/submucosa only) | | **Non-caseating granulomas** | 30–50% | Absent | | **Ulcer morphology** | Fissuring, linear, aphthoid | Superficial, crypt abscess | | **Distribution pattern** | Skip lesions (discontinuous) | Continuous from rectum | | **Anatomical extent** | Mouth to anus (any segment) | Colon and rectum only | | **Fistula/stricture formation** | Common | Rare | | **Intraepithelial lymphocytes** | Increased | Increased | | **Chronic inflammation** | Yes | Yes | **Clinical Pearl:** Transmural inflammation in Crohn disease allows the inflammatory process to penetrate all bowel layers, leading to penetrating ulcers, fistulas, and strictures — complications that are uncommon in ulcerative colitis because inflammation is superficial. **Warning:** A common exam trap is to confuse which disease has transmural involvement. Remember: **Crohn = transmural; UC = mucosal/submucosal**.